Storage devices employing semiconductor elements are roughly classified into two categories: a volatile storage device that loses stored data when power supply stops, and a non-volatile storage device that retains stored data even when power is not supplied.
A typical example of a volatile storage device is a dynamic random access memory (DRAM). A DRAM stores data in such a manner that a transistor included in a memory element is selected and electric charge is stored in a capacitor.
When data is read from a DRAM, electric charge in a capacitor is lost on the above-described principle; thus, another writing operation is necessary whenever data is read out. Moreover, a transistor included in a memory element has a leakage current (off-state current) between a source and a drain in an off state and electric charge flows into or out of a capacitor even if the transistor is not selected, whereby a data holding period is short. For that reason, another writing operation (refresh operation) is necessary at predetermined intervals, and it is difficult to sufficiently reduce power consumption. Furthermore, since stored data is lost when power supply stops, an additional storage device utilizing a magnetic material or an optical material is needed in order to hold the data for a long time.
Another example of a volatile storage device is a static random access memory (SRAM). An SRAM retains stored data by using a circuit such as a flip-flop and thus does not need refresh operation. This means that an SRAM has an advantage over a DRAM. However, cost per storage capacity is increased because a circuit such as a flip-flop is used. Moreover, as in a DRAM, stored data in an SRAM is lost when power supply stops.
A typical example of a non-volatile storage device is a flash memory. A flash memory includes a floating gate between a gate electrode and a channel formation region in a transistor and stores data by holding electric charge in the floating gate. Therefore, a flash memory has advantages in that the data storing time is extremely long (almost permanent) and refresh operation which is necessary in a volatile storage device is not needed (e.g., see Patent Document 1).
However, a gate insulating layer included in a storage element deteriorates by tunneling current generated in writing, so that the memory element stops its function after a predetermined number of writing operations. In order to reduce adverse effects of this problem, a method in which the number of writing operations for memory elements is equalized is employed, for example. However, a complicated peripheral circuit is needed to realize this method. Even when such a method is employed, the fundamental problem of lifetime is not resolved. In other words, a flash memory is not suitable for applications in which data is frequently rewritten.
In addition, high voltage is necessary for holding electric charge in the floating gate or removing the electric charge, and a circuit therefor is required. Further, it takes a relatively long time to inject or remove electric charge, and it is not easy to perform writing and erasing at higher speed.